This invention relates to a method of making and a structure for smooth silicon carbide films and particularly to silicon carbide films used in X-ray masks and vacuum windows.
The X-ray mask is a critical structure in X-ray lithographic proximity printing. In general, X-rays from a point source of soft X-rays are shadowed by a heavy element mask consisting of a pattern absorber layer supported on a flat membrane, or pellicle, which is relatively transparent to the radiation. At the present time, the pellicle supporting the pattern absorber layer is made of a thin, inorganic material which minimizes X-ray attenuation, and yet remains mechanically stable to minimize distortion caused by stresses in the patterned absorber layer. Typically the pellicle is stretched across a stiff, flat ring whose expansion coefficient closely matches that of silicon. This stretching process flattens the substrate and stiffens it against bending and breaking. Factors important to mask fabrication are dimensional stability, absorber line-edge profile, and defect density. Also X-ray mask fabrication requires many steps which are similar to those used in wafer processing. As in wafer processing, these processing steps contribute to the defect density in the X-ray mask. Hence, defect density remains a highly critical problem.
Presently available pellicles are fabricated by the deposition of sequential layers of boron nitride and polyimide onto a sacrificial silicon substrate. The boron nitride is generally formed by the reaction of ammonia and diborane in a suitable chemical vapor deposition process. Typically a layer of filtered polyimide is spun on top of the boron nitride from a liquid source in order to cover small defects. However, many defects still remain at the polyimide surface.
In addition to boron nitride, silicon carbide has recently been deposited by chemical vapor deposition (CVD) onto silicon to form X-ray pellicles. This film is chemically inert and has excellent mechanical stability and strength, much stronger than boron nitride. Furthermore, the coefficient of expansion of silicon carbide can be closely matched to that of silicon. It is these properties of silicon carbide that make it an ideal mask support material. However, CVD of silicon carbide also has some inherent disadvantages. If the silicon carbide is deposited directly onto the wafer surface, the film tends to have a large number of defects. Also, for direct deposition on silicon, the deposition parameters required for optimum film smoothness do not necessarily coincide with the conditions required for optimum stress in the membrane.
Furthermore, vacuum windows used, for example, in electron beam addressed liquid crystal displays require deposition of a silicon carbide film directly onto a silicon wafer, thus also requiring a smooth silicon carbide surface. Additionally, the silicon carbide film must be thick enough to stop electrons produced from electron beam bombardment and thin enough to have low lateral heat loss. A silicon carbide film typically two microns thick is required to stop electrons in the range of 15 KEV to 20 KEV. However, when deposited to this thickness the surface roughness of the silicon carbide film is increased and correct alignment of the molecules of the liquid crystal on such a rough silicon carbide surface is difficult.
Therefore, to alleviate the present disadvantages of using silicon carbide films for the fabrication of X-ray masks and vacuum windows, a new mask structure has been developed. The present invention presents this structure and a method for its construction.